A fourth consecutive poll has found Democrat Elizabeth Warren ahead of Republican US Senator Scott Brown in a race that could tip the balance of power in the US Senate.

The new poll by WBUR found that 45 percent of likely voters back Warren, while Brown is the choice of 40 percent. The survey found that 12 percent were undecided. The poll of 507 likely voters has a margin of error of plus or minus 4.4 percentage points, the radio station said in a statement.

Earlier this week, a Suffolk University/WHDH-TV (Channel 7) survey had Warren at 48 percent and Brown at 44 percent, a finding that reflected a shift toward Warren. In May, a Suffolk/WHDH survey had Brown at 48 percent and Warren at 47 percent.

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The candidates are set for their first debate Thursday at the WBZ-TV studios. Additional debates are set for Oct. 1 in Lowell, Oct. 10 in Springfield, and Oct. 30 in Boston.

The WBUR poll was conducted Sept. 15-17 by the MassINC Polling Group. The poll found that Warren appears to be gaining the respect of prospective voters while Brown’s standing has eroded. In the survey, 39 percent of likely voters believed Warren “will stand up for regular people when in the Senate,” an improvement from 30 percent from a poll in February.

On the same question, Brown’s support dropped to 29 percent from 33 percent.

In what the station described as a sign that Warren’s campaign themes seem to be resonating with voters, the poll found that 35 percent of voters view Warren as the candidate who best “understands the needs of middle-class families.” Only 27 percent said that phrase described Brown.

Brown drew strong support when voters were asked about the candidates’ connections to Massachusetts. Forty-four percent of those surveyed said Brown, a Wakefield native who now lives in Wrentham, had strong ties to the state. For Warren, an Oklahoma native who moved to Massachusetts to teach at Harvard Law School, just 13 percent of the voters said she had a deep connection to Massachusetts.

While both campaigns have released negative ads this month, Brown is considered by 35 percent of the voters to be running a more positive campaign compared to 21 percent for Warren, the poll found.

Also this week, Springfield Republican survey, taken by the Western New England University Polling Institute, put Warren in the lead 50 percent to 44 percent, just outside the poll’s 4.6 percentage point margin of error. Another survey, by Public Policy Polling, a Democratic leaning group, showed Warren at 48 percent and Brown at 46 percent, within that poll’s 3.3 percentage point margin of error.